Let’s see one of them https://freedns.afraid.org. After you register, go to Dynamic DNS link (on the left top box-for members).
Add your host with type A and subdomain and domain what you like as host.

Now there’s going to be a list of your host names. Right click on the Direct Link and copy the link. You should keep the alpha-numeric key. The address will be something like http://freedns.afraid.org/dynamic/update.php?[alpha-numeric-key]